To support the mobility of a communication/mobile device in a wireless network, there are procedures in both media access control (MAC) layer and network layer in a network configuration trying to maintain the upper level on-going communication uninterrupted. The MAC layer handover occurs when a mobile device leaves one access point (AP) and enters another AP. The network layer handover occurs when a mobile device switches from one network domain into another network domain under a new internet protocol (IP) address, which is mainly based on mobile IP techniques. If a mobile device needs to switch across different network domains, in other words, when a mobile device leaves the home network and enters a foreign network, this mobile device has to obtain a new IP address in the foreign network as the care of address (CoA), and at the same time, needs to register the new IP address in the foreign network at the home network, so that the home agent can automatically relay the transmission packets for the mobile device to the new IP address in the foreign network and keeps the ongoing communication uninterrupted. With the current internet operation in reality, the prevalence of mobile IP is still limited.
FIG. 1 illustrates the procedure involved for a mobile device switching from access point AP1 to access point AP2. When the mobile device detects the signal-to-noise ratio (SNR) of AP1 is lower than its cell search threshold, an automatic scan mechanism is activated for a new access point. When the signal magnitude of AP2 is stronger than the signal magnitude of AP1 by a certain value (ΔSNR), the mobile device automatically handovers to the new AP2.
FIG. 2 shows an example for a session initiation protocol (SIP) based mobility management in a wireless network. Referring to FIG. 2, there are two SIP clients in communication session, the mobile device MS in A1 network domain 201 and the corresponding node CN in the B1 network domain 202. While the mobile device MS switches from A1 network domain 201 to a new A2 network domain 203, a conventional mobility management procedure includes the following steps.
Step a. While switching network domains, the mobile device MS keeps on detecting and determining if the signal from AP1 has dropped below the cell search threshold. If yes, then mobility management procedure jumps to step b, otherwise, repeats the current step a.
Step b. The mobile device MS initiates a scan mechanism for a new access point and detects a stronger signal from the AP2 of the network domain A2. With MAC layer connection, mobile device MS connects to the AP2 of the A2 network domain. At this step, the communication status of the mobile device MS and the corresponding connection node CN is temporarily disconnected.
Step c. The mobile device MS. via AP2. obtains an IP address within network domain A2 from the server for dynamic host configuration protocol (DHCP). At this step, the communication status of the mobile device MS and the corresponding connection node CN is still temporarily disconnected.
Step d. The mobile device MS registers a new IP address in network domain A2 at the SIP proxy/registrar server, so that the other clients can reach the mobile device via the SIP proxy/registrar server. At this step, the communication status of the mobile device MS and the corresponding connection node CN is still temporarily disconnected.
Step e. The mobile device MS issues an SIP INVITE message to the corresponding connection node CN to re-establish the communication. Only with successful SIP INVITE process can the previously broken connection be re-established.
Whenever a mobile device switches across different domains and needs a new IP address, the fore going handover procedure takes about more than 10 seconds, which includes time for MAC layer, network layer and application layer handovers. For networks providing real-time voice services, due to the lengthy network domain handover time, the communication interruption is too long to be of an acceptable service quality.
When a mobile device moves across different network domains, the sources of the communication interruption come from affected interfaces for MAC layer, network layer and application layer. In the whole domain transition process, the time for service interruption includes three categories as shown in FIG. 3. The time when the mobile device stops connecting with AP1 till the time the mobile device connects to a new AP2 in a new network domain is defined as the MAC layer handover time, denoted as Δ1 in FIG. 3. Secondly, the time when the mobile device starts connecting to the new AP2 till the time the mobile device obtains a new IP address via the DHCP server in the new network domain is defined as the network layer handover time denoted as Δ2 in FIG. 3. Finally, the application layer handover time is defined as the time for re-establishing the communication service with the new IP address in the application layer, which includes the mobile device MS registers a new IP address in network domain A2 at the SIP proxy/registrar server, the mobile device MS issues an SIP INVITE message to the corresponding node CN to re-establish the communication, till the completion of the new communication connection, denoted as Δ3 in FIG. 3.
The total latency for service interruption from mobile device inter-domain handover is the sum of Δ1+Δ2+Δ3. In other words, for an efficient mobile communication service, the total handover interruption time, Δ1+Δ2+Δ3, can be minimized by speeding up the connection time to a new AP in the MAC layer, by minimizing the time for obtaining a new IP address or even by pre-assigning an IP address in the network layer, and by avoiding the repetition for re-establishing the new connection in the application layer.
The US patent publication 2002/191593 disclosed a method and apparatus for session signaling and mobility management in communication systems. In this method, the session signaling and mobility management is accomplished by SIP integrated with the mobile IP. In a SIP-based configuration, each cell area includes many modules, such as session signaling server module, mobility agent modules, etc. When a mobile device switches to a new cell area, this mobile device first registers itself at the session signaling server module with attached mobile device information before the transition. In the mean while, the session signaling server module informs the mobility agent module and the original mobility agent module for the requesting mobile device of a handover procedure. This handover procedure includes the following steps. The original session signaling server module issues a request message to the session signaling server module of the new cell area, and switches the associated state information of the requesting mobile device to the modules in the new cell area. After receiving the request message, the session signaling server module in the new cell area replies a response message back to the home session signaling server node with the location of the mobile device.
This technique disclosed the method for inter-domain handover, and assures a seamless session after the mobile device switches to the new cell area and obtains a new IP address. This technique, however, still cannot resolve the overhead for requesting a new IP address after the mobile device switches to the new cell area. During the cell area handover, packet loss remains an issue. This technique still cannot fulfill the data transmission quality required by most real-time multi-media service applications.
In the patent WO0131472, with respect to the 3G all IP network environment, disclosed a mobility management protocol to support the real-time and non-real-time multi-media application programs for mobile devices. This technique utilizes and extends the SIP in order to provide domain handover and subnet handover in such a way that a user can always access Internet via the mobile device from any location while roaming in different 3G all IP networks. This technique also supports the portability for the application programs of the transmission control protocol (TCP), so that there is no need to modify the TCP. When a mobile device switches to a new network domain, with SIP INVITE and INFO method, IP address binding and user authentication can both be accomplished. With SIP registration, a new IP address can be obtained from a DHCP server, and the TCP connection on a mobile platform can also be maintained with a media agent named SIP_EYE.
The technique disclosed in this patent starts the process of new registration as well as the informing the corresponding node (CN) and edge router controller of re-establishing new connection and transient tunnel after the communication device switches to a new domain. Consequently, before the new connection is established, the packet loss issue still exists. This technique saves the overhead for requesting a new IP address from the new DHCP, the latency is still needed for completing the registration at the SIP server. The data transmission quality of such technique still cannot fulfill the requirement for real-time multi-media service applications.
A U.S. Pat No. 6,766,171 disclosed a method and apparatus for a duplex soft handover in a network environment with no carrier control. By utilizing the media gateways installed in different network environments, this technique allows a duplex soft handover operation leaving the signaling of the carrier network untouched. In such a configuration, a mobile device always needs to establish a connection via a media gateway before establishing connection with target node. On detecting a mobile device moving into a transition area, a media gateway informs the mobile device to establish a new connection. As the mobile device continues moving, its media gateway automatically handovers into the new connection, and disconnects the original communication.
The technique disclosed in this U.S. Pat. No. 6,766,171 requires the mobile devices to have two receivers. This results in not only higher manufacturing cost but also more power consumption. These two issues affect the evaluation for designing mobile device.
In conclusion, the mobility management in most conventional wireless networks is supported by mobile internet protocol (mobile IP) with home agent and foreign agents to avoid the handover in the application layer procedure. With mobile IP available in lower layers, the upper layer application procedure can minimize the time in Δ3, but still needs overhead for registering a new foreign agent at the home agent. For choosing mobile IP as the network layer protocol, the most criticized issue is triangular routing problem. Besides, the prevalence of mobile IP is still limited. The cost for installing facilities for mobile IP not only includes deploying many home agents and foreign agents, but also involves the fact that the mobile IP needs two IP addresses in most of time in services. These disadvantages are even more unfavorable in the IPv4 environment in which IP address is a rare resource. From the perspective of the whole service latency, the mobile IP approach although reduces the time in Δ3, other mechanism is still needed to minimize both Δ1 and Δ2.